In order to maintain a long battery life and save power, user equipments, e.g., in Wideband Code Division Multiple Access (WCDMA) systems can be switched from an active state to a power saving state. The battery life is maximized by switching to the power saving state frequently and quickly. However, if this switching is very frequently performed, the user equipments will often find themselves in the power saving state when they need to transmit data. If the switching is slow, it means that the user-perceived performance will suffer from the inertia that occurs every time a new transmission is commenced.
In the work item “Signalling enhancements for Circuit-Switched (CS) and Packet-Switched (PS) Connections” targeted by the 3rd Generation Partnership Project (3GPP) for Release 7, some enhancements to optimize the signalling have been proposed. In general the focus has been in optimizing the connection set-up time from idle mode.
Also within the scope of High speed packet access (HSPA) evolution it is desirable to reduce the time for transitions from the power saving state. The power saving state can be either CELL_PCH or URA_PCH. The following description mainly focuses on the CELL_PCH state, but similar principles could be applied for the URA_PCH state as well.
The current sequences for an UE initiated transition from the CELL_PCH state to the CELL_DCH state are illustrated in FIGS. 2a and 2b. FIG. 2a illustrates the sequence for a normal transition, while FIG. 2b illustrates the sequence for a direct transition to DCH. In direct transition the information contained in the Radio bearer reconfiguration message is already indicated by the Cell update confirm message (most likely using default parameter values).
In FIG. 2a the radio network controller RNC sends a paging message 210 to the user equipment UE. After a power ramping procedure 215 the UE sends a Cell Update message 220 to the RNC, which is confirmed in a Cell Update Confirm message 225 back to the UE. Then, the UE sends a UTRAN Mobility Information Confirm message 230 to the RNC. The RNC sends a Downlink User Plane Initialisation message 235 to the Node B whereas the Node B sends an Uplink Plane Initialisation message 240 to the RNC and provides a Downlink Transmitter Power On (DL Tx Power On) indication 245 to the UE. Then, the RNC sends a Radio Bearer Reconfiguration message 250 to the UE. If synchronisation in the UE is achieved 255, the UE provides an Uplink Transmit Power On (UL TX Power On) indication 260 to the Node B. If synchronisation has been achieved in the Node B 265, it sends an E-DCH scheduling grant message 270 to the UE. Finally, the UE sends a Radio Bearer Reconfiguration Complete message 275 to the RNC. The time TUE denotes the time interval from which the UE has received data until the Radio Bearer Reconfiguration Complete message has been transmitted to the RNC.
Accordingly, in FIG. 2b the radio network controller RNC sends a paging message 210 to the user equipment UE. After a power ramping procedure 215 the UE sends a Cell Update message 220 to the RNC, which is confirmed in a Cell Update Confirm message 225 back to the UE. Then the RNC sends a Downlink User Plane Initialisation message 235 to the Node B whereas the Node B sends an Uplink Plane Initialisation message 240 to the RNC and provides a Downlink Transmitter Power On (DL Tx Power On) indication 245 to the UE. If synchronisation in the UE is achieved 255, the UE provides an Uplink Transmit Power On (UL TX Power On) indication 260 to the Node B. If synchronisation has been achieved in the Node B 265, it sends an E-DCH scheduling grant message 270 to the UE. Finally, the UE sends a UTRAN Mobility Information Confirm message 280 to the RNC. The time TUE denotes the time interval from which the UE has received data until the UTRAN Mobility Information Confirm message has been transmitted to the RNC.
In both sequences, the Cell Update Confirm message is transmitted on the CCCH over the Forward Access Channel (FACH). In current art, it is not possible to use any other channel than CCCH for the transmission of the Cell Update Confirm message as the Cell Update Confirm message contains the identifier (C-RNTI or H-RNTI) that is needed to set up dedicated connections. C-RNTI is defined for the UE in the particular cell when it is using dedicated channels, while H-RNTI is for HS-DSCH.
The procedures described above in FIGS. 2a and 2b take significant amount of time due to the fact that it is necessary to use CCCH and FACH in order to receive the H-RNTI. It is further necessary to receive information on the allocated downlink and uplink channel (either by using Radio Bearer Reconfiguration or directly in the Cell Update Confirm) before initiating the synchronization procedure.
The main contributions to the switching time come from the transmission of the Cell Update message (including the power ramping), transmission of the Cell Update Confirm message (and, possibly, the UTRAN Mobility Information Confirm and Radio Bearer Reconfiguration message) and from the synchronization. The following shortly describes some known ways to reduce these delays:
1) The Cell Update message could be transmitted faster by either reducing the power ramping or by increasing the transmission rate of the RACH channel. However, these modifications would require significant changes to the existing random access procedure.
2) The data rate of the FACH channel could be increased or the Transmission Time Interval (TTI) shortened in order to reduce the time needed for the Cell Update Confirm message. However, as the size of the Cell Update Confirm message is not very large, an increase of the data rate does not significantly improve the total switching time. Changing of the TTI would require complete redesign of the FACH channel, which is disadvantageous.
3) The synchronization could be done faster by increasing the amount of hardware resources for this purpose. However, this solution increases the manufacturing costs of both the base station and the terminal, which is not desirable.
Thus, it is desirable to shorten the time needed for performing the above-described UE-initiated state transition procedures, in particular for transitions from a power saving state, e.g. the CELL_PCH state, to the CELL_DCH state, while avoiding the disadvantages mentioned above.